Historically, servo controls on an axial piston closed circuit pump have used a transverse servo bore and piston with one or two servo covers to seal the servo cavity. These servo covers add cost since they result in additional parts, additional machining to mating parts, and potential leak paths.
The other issue has been package size. Control resolution performance improves with a longer moment arm and the corresponding longer servo piston travel. In a traditional transverse servo piston arrangement, increasing servo piston travel increases package size, especially in the width dimension. With an axial servo piston, however, the longer servo piston travel can be accommodated while optimizing the width and without increasing the length of the pump.
It is therefore a principal object of the invention to provide an axial servo control having a housing and a piston block with a rotatable axes and a servo piston bore with a single cover attached to the housing to close adjacent open ends in both the housing and the piston bore.
A variable displacement hydraulic power unit has a housing and a rotating piston block with a rotatable axis and a servo piston bore having a reciprocal piston therein with the bore extending in a direction parallel to the rotatable axis of the rotating piston block. The housing and servo piston each have adjacent open ends. A single integral cover is attached to the housing to close the open ends of both the housing and the piston bore.